You must enable the assistive technology services before you are able to use many of the accessibility tools available.
Choose System ▸ Preferences ▸ Assistive Technologies.
Select Enable assistive technologies and then press Close and Log Out.
Log back in. The assistive technology services will be started automatically.
Once basic assistive technology support is enabled, it is possible to start accessibility services such as the screen reader, screen magnifier and on-screen keyboard.